The present invention relates to a modular bridge, particularly although not exclusively for military use. It also relates to a module for use in such a bridge.
A typical modular bridge comprises a plurality of like modules which can be transported separately to the place they are needed and then linked together on site. The modules forming the central section of the bridge generally have parallel upper and lower chords, the upper chord comprising or bearing a surface for carrying traffic, and the lower chord being arranged to withstand the tension load resulting from the dead weight of the bridge plus the weight of the vehicles passing over the bridge. At each end of the bridge there are special ramped end modules to enable vehicles to drive onto and off of the bridge.
With the increasing use of containerisation in recent years, it has become more and more desirable for the individual bridge modules to be of such a size that they can be transported in standard ISO containers. To this end, foldable modules have been developed which consist of a central deck structure having, at either side, foldable main girder structures which in use will bear the loads to be applied to the bridge. The main girder structures are hinged to the edges of the central deck structure so that they can be pivoted between a use position in which they form lateral extensions of the central deck structure, and a folded position in which they are rotated through about 90.degree. to tuck neatly beneath the central deck structure for transport.
In this way, a module having a four meter wide roadway (that is, the width of the central deck structure plus the width of the two lateral extensions formed by the main girder structures in use) will fold longitudinally into a compact envelope not exceeding eight feet in width and four feet in height. Thus, two modules may be stacked on top of each other on a flat rack or pallet within the dimensions of a standard ISO container.
A difficulty with this arrangement is that it limits the depth of the main girder structures, and thus the load bearing capacity of the bridge. When the main girder structures have been folded inwardly through 90.degree., they must of course fit within the eight feet envelope of the ISO container, so that the depth of each girder cannot be greater than four feet. For bridge spans of greater than one hundred feet (thirty two meters) for a sixty tonne vehicle load, the structural efficiency of the girders will be severely limited unless the depth can be increased. This puts a fundamental limit on the span that a bridge of this type can have for a given load; or, to put it another way, it limits the load that the bridge can carry for a given span.